At least six people were hospitalized after the police responded to a shooting at the student union building on the campus of Florida State University in Tallahassee on Thursday, officials said. One person was taken into custody, according to the police, and the university said the threat had been “neutralized.”

The Tallahassee Memorial HealthCare system said it had received six patients: one in critical condition and five others in serious condition.

Gunshots were reported at the student union building shortly before noon, and the university soon issued a shelter-in-place alert. Students were advised to remain indoors and not to return to the student union to retrieve personal items. The building and its surrounding area were still “an active crime scene,” according to the university.

The gunfire sent students and staff seeking cover. A professor, Chris Coutts, said he was “baffled” as he walked to his car after having sheltered in place. “This is the second one since I’ve been here,” he said, referring to a 2014 shooting in which a gunman wounded three people in the library.

Officers began evacuating academic buildings soon after the shelter-in-place alert went out. Local and state police officers were on campus escorting students from the buildings near the student union.

The F.B.I. in Jacksonville, Fla., said in a statement on social media that it was assisting with the response. The governor, Ron DeSantis, said on social media that state law enforcement was “actively responding.”

University officials canceled all classes, events and business operations for the rest of the week. All athletic events are canceled through the weekend. The last day of classes is next Friday.

School officials say that law enforcement officials have “neutralized the threat” on campus. Officials are still investigating, and the student union and surrounding area “are still considered an active crime scene,” but the university lifted the shelter-in-place order.

The Florida State chapter of the College Democrats condemned the shooting in a statement, calling it a “completely preventable act of senseless violence.” It added, “This is not normal; this is not acceptable.”

All classes and business operations on the main campus are canceled through Friday, the university said. It also canceled all athletic events through the weekend.

The university said students were now allowed to return to their residence halls, but advised that they stay indoors while emergency workers continue to investigate.

A student who was in the student union at Florida State University, near the scene of the shooting, said he was momentarily confused by the sound of gunfire but quickly took cover.

“I’m still pretty disoriented from it,” said Victor Castillo, 20, a sophomore studying business management. He had been eating lunch in the building when he heard what sounded like explosions, then hid under his stool.

“All of a sudden, I hear boom, boom, boom,” Mr. Castillo said. “My ears started ringing; I didn’t know what was going on. I didn’t think it would happen here.”

Mr. Castillo sheltered in place until the police evacuated the building.

Chris Coutts, 51, a professor of urban planning at the school, said he was “baffled” by what had happened. “This is the second one since I’ve been here,” he said as he walked to his car after having sheltered in place. He was referring to a 2014 shooting in which a gunman wounded three people in the university library.

President Trump said that he had been briefed on the shooting. “It’s a shame, a horrible thing, it’s horrible that things like this take place,” he said.